1. Field of the Invention
The present invention relates to a heat dissipating substrate suitable for use as electronic devices such as semiconducting lasers and diodes, particularly relates to a highly-oriented diamond film heat dissipating substrate with improved radiating characteristics.
2. Prior Art References
Semiconductor devices are becoming increasingly high speed and high density. According to such a trend, heat generation of semiconducting elements and heat dissipation thereof are becoming important problems.
One of the approaches to these problems is the improvement of thermal conductivity of the substrate itself on which semiconducting elements are formed. The substrate material used for integrated circuits (ICs) and semiconducting lasers are usually Al.sub.2 O.sub.3, but this material has a disadvantage that its thermal conductivity is not high enough.
Diamond has the highest thermal conductivity among various materials. Although attempts have been made in some fields where diamond is used as a heat dissipating substrate, this type of heat dissipating substrate utilizing diamond can not be widely used because they are natural 2A-type diamond of which thermal conductivity is high but they have limited availability and high price. Another problem associated with natural diamond is heterogeneous thermal conductivity due to nonuniform distribution of nitrogen, and hence it is difficult to supply substrates with a reproducible thermal property. Further, diamond is a very hard material and difficult to fabricate in the manufacturing process of heat sinks, therefore manufacturing cost is very high. Heat dissipating substrates have been developed using artificially synthesized diamonds (Japanese under provisional Publication sho 60-12747). However this type of heat dissipating substrate, even though artificially synthesized diamond is used, still has a disadvantage that extremely high pressure is needed for its synthesis, very complex manufacturing process is required and its manufacturing cost is high.
Under these backgrounds, a heat dissipating substrate has been developed with diamond grown by chemical vapor deposition (CVD). In those heat dissipating substrates, the substrates contain impurity to improve its electric conductivity (Japanese under Provisional Publication sho 61-251158) or the diamond heat sink is obtained by growing diamond on a plate-like substrate by CVD in order to reduce a polishing process which may significantly increase the manufacturing cost (Japanese under Provisional Publication 2-273960).
However, diamond films formed by CVD as represented by plasma CVD and hot filament CVD methods consist of polycrystalline diamond except for the diamond film formed on a cubic BN (referred hereinafter as to cBN) substrate. In these polycrystalline diamond films, crystal facets appear irregularly on the film surface and there exists a considerable irregularity of about 0.5 .mu.m between the planes of adjacent crystals. Therefore, when a polycrystalline diamond film is used for a heat sink, the film must be polished to enhance the thermal conductance. However, the manufacturing cost increases by the polishing process.
The thermal conductivity of polycrystalline diamond films is about 10 W/cm.multidot.K which is lower than that of natural 2A-type diamond, and therefore they are inferior to natural diamond in heat dissipation characteristics. Likewise, a single crystal diamond grown on a cBN substrate is disadvantageous because is a very hard material and therefore its machining is difficult. In addition, inexpensive single crystal cBN is not available. Therefore, in so far as diamond heat dissipating substrates are concerned, polycrystalline diamond films and not single crystal diamond films must be used.